In the field of Natural Language Processing, in order to work out a thematic representation system of general knowledge, methods relying on thesaurus have been used for about twenty years. A thesaurus consists of a set of concepts which define a generating system of a vector space modelling general knowledge. These concepts, often organized in a treelike structure, constitute a fundamental, but completely fixed tool. Even if the concepts evolve (we think for example of the technical fields), a thesaurus as for it can evolve only at the time of a particularly heavy process, because it requires the collaboration of human experts. After detailing the characteristics which a generating system of the vector space of knowledge modelling must have, we define the "basic notions". Basic notions, whose construction is initially based on the concepts of a thesaurus, constitute another generating system of this vector space. We then approach the determination of the acceptions expressin...